1. Field of the Invention
This invention pertains to 3-dimensional imaging. More particularly, this invention relates to sectioning microscopes which reject light from all out-of-focus planes and form a sharp sectioned image of the object specimen.
2. Background of the Prior Art
The demand for 3-dimensional (3-D) imaging is dramatically increasing in such fields as biomedicine, clinical diagnosis, and ophthalmology. The sectioning, or depth discrimination capability, of a microscope can be characterized by the variation of the total power in the image of a point source as it is scanned through the focus. Conventional microscopes are nonscanning parallel processing systems which image the entire object field simultaneously and do not have imaging sectioning capability. As shown in FIG. 1, the power received from plane 1 (out of focus) is the same as that received from plane 2 (in-focus). The light from unfocused portions of the object falls onto the image plane along with the light from the focused portion of the object, thereby contaminating the focused image with a strong blurred background. The total power in a conventional microscope is constant in the image of a point source during defocus. 3-D data (both the axial and lateral light energy distribution) cannot be obtained from a conventional microscope.
In recent years, scanning confocal microscopy has rapidly developed to meet the 3-D imaging demand. Confocal microscopes can well reject the light from all out-of-focus planes and form a very sharp sectioned image of the object. In a standing confocal microscope, the point spread function can be expressed as the fourth power of a coherent point spread function. The curve representing the integrated intensity variation with the focus is shown in FIG. 2 and demonstrates that the confocal microscope possesses good depth discrimination capability. When the defocus is equal to the focal depth (u=3), the power drops to 70%. At twice the focal depth (u=6), the power drops to 20%. However, to gain this sectioning capability, the confocal microscope has to sacrifice image acquisition speed.
A confocal microscope uses a point light source and a point detector, and therefore the image data must be scanned pixel by pixel, which is undesirably slow. First, the available photon number for each pixel is severely limited by the scanning speed. For example, for an image width 1000.times.1000 pixels, the photon collecting time for each pixel is only one millionth of the frame time. Second, a complicated and expensive scanning device (either mechanical or acoustic), has to be used. Third, the instrument requires critical alignment. Consequently, confocal microscopes are inefficient, complicated, expensive and difficult to maintain. Confocal microscope technology is explained in T. Wilson, "Confocal Microscopy" Academic Press (1990) incorporated herein by reference.
A microscope which can reject light from all out-of-focus planes and can form a sharp sectioned image of the object without the inefficiencies of confocal microscopes caused by complex and expensive scanning and alignment devices would be of great benefit.